Technologies using hydrogen as an energy source have been recently remarkably developed as seen in the growth of the fuel cell. In this field, countermeasures against hydrogen have been long investigated with respect to materials per se for a storage container or piping in connection with a high pressure hydrogen storage technology. The negative effect of hydrogen on a metal material has been long studied in the field of corrosion. For example, hydrogen gas generated by a cathode reaction in a corrosive solution is adsorbed on the tip of a stress concentrated source, such as a defect, an inclusion and a deposit, or penetrates and accumulates in a material near the defect embrittling the area, so that a crack propagates in an element leading to destruction. Recently the problem of the hydrogen embrittlement of a metal material has drawn special attention, namely hydrogen penetrates into a metal material, such as steel, to lower the ductility of the metal material. Progress of the hydrogen embrittlement may bring a serious consequence such as fracture of the metal material. Such fracture of a metal material due to the hydrogen embrittlement is called as a delayed fracture phenomenon. The delayed fracture is also called as static fatigue, since a sudden brittle fracture can break out in a high strength element placed under a static tensile stress for a certain period of time. It is believed that such delayed fracture of a high strength element is caused by hydrogen penetrated into the element at the fabrication stage or from the environment during the usage. Since hydrogen penetrates easily to a metal element having higher concentration of atomic vacancy induced by plastic deformation, a fracture, namely hydrogen embrittlement occurs concentratively in the vicinity of tensile stress concentrated area, such as an area with a screw or a corrosion pit. The occluded hydrogen in a metal, especially steel, has generally little effect on the yield strength or the tensile strength, but is of the nature of deteriorating the ductility and tenacity. Therefore, the higher strength a metal element has, the higher susceptibility to the hydrogen embrittlement the element has, and therefore especially the high strength steel needs close attention to hydrogen.
There has been little research or investigation on the hydrogen embrittlement from the tribological viewpoint. But in technologies concerning use of hydrogen as an energy source such as fuel cell, transportation of hydrogen is necessary, and therefore mechanical elements for transportation become necessary inevitably. A typical example is a compressor, in which such tribological elements as a rolling bearing and a sliding bearing are used. Consequently, countermeasure against the hydrogen embrittlement for those mechanical elements and metal materials is important, but currently little countermeasure has been taken.
Meanwhile also in the field of rolling bearings for automobile electrical and auxiliary devices, the hydrogen embrittlement has been a problem for long, and to cope with the problem the properties of grease used for them have been improved. For example, it has been proposed to add a passivation oxidant in the grease to inhibit the catalyst activity of the fresh surface created by wear by oxidizing the metal surface, so that hydrogen generation by decomposition of the lubricant can be inhibited (e.g. Patent Document 1). Another proposal is to use a phenyl ether-based synthetic oil as a base oil of a grease, so that hydrogen generation by decomposition of the lubricant can be inhibited (e.g. Patent Document 2). Another proposal is addition of a specific thickening agent, a passivation oxidant and an organic sulfonate to a specific base oil (e.g. Patent Document 3). It has been proposed to add an azo compound absorbing hydrogen to a grease to be filled in tribological materials or various elements and in bearings to be used in locations where water may enter easily (e.g. Patent Document 4). A grease composition for a long-lasting rolling bearing has been proposed, which comprises a fluorinated polymer fluid as a base oil, polytetrafluoroethylene as a thickening agent and an electroconductive material, and which does not cause flaking by hydrogen embrittlement, even if attacked by water (e.g. Patent Document 5). All of these measures are, however, against a small amount of hydrogen generated by decomposition of grease, etc. and are neither disclosing nor indicating measures to suppress a flaking, a hydrogen embrittlement-caused fracture or a hydrogen embrittlement-caused flaking in the hydrogen existing environment, in which hydrogen is actively introduced.
[Patent Document 1] JP-A-03-210394
[Patent Document 2] JP-A-03-250094
[Patent Document 3] JP-A-05-263091
[Patent Document 4] JP-A-2002-130301
[Patent Document 5] JP-A-2002-250351